Method for printing a substrate with radiation curable ink, and an ink suitable for application in the said method

ABSTRACT

A method for printing a substrate with radiation curable ink supported on a substrate which includes the steps of providing an inkjet print head at an operating temperature, jetting droplets of the curable ink from the inkjet print head onto the substrate, controlling the interaction between the ink and the substrate, and then curing the ink received on the substrate by directing radiation toward the substrate, wherein the controlling of the interaction is provided for by applying an ink comprising a carrier composition and an agent that is able to reversibly gel the carrier composition, which agent is soluble in the carrier composition at the operating temperature of the inkjet print head and gels the ink when it is received on the substrate. Also provided is an ink for enabling the use of the method and a printer adapted for applying the method.

This application claims priority benefit of European Patent ApplicationNo. 05107997.8 filed Sep. 1, 2005, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention pertains to a method for printing a substrate withradiation curable ink which comprises providing the substrate on asupport, providing an inkjet print head at an operating temperature,jetting droplets of the curable ink from the inkjet print head onto thesubstrate, controlling the interaction between the ink and thesubstrate, and then curing the ink received on the substrate bydirecting radiation toward the substrate. The present invention alsopertains to an ink suitable for application in the present method and aprinter for applying the method.

Such a method is known from European patent EP 1 349 733, issued Nov.10, 2004 as well as from U.S. Pat. No. 6,145,979 issued Nov. 14, 2000.Radiation curable inks have become widely considered in recent years asthe ink of choice for printing a wide variety of non-coated, non-poroussubstrates. In particular, UV-curable inks or electron beam curable inksare well known in the art. In general, but not necessarily, these inksare based on a carrier composition comprising ethylenically unsaturatedmonomer or oligomeric binders that polymerize under the influence ofradiation to form a cured binder. The use of radiation curing enablesthe ink to quickly cure (commonly considered as “instant drying”)without the need to drive off large quantities of water or othersolvents. As a result, radiation curable inks can be used in high speedinkjet printers that can achieve production speeds of over 100 squaremeters per hour. It is a common desire that the same printer can be usedto print a wide variety of substrates, each image printed being of thesame high quality regardless of the type of substrate being used.However, it is known from the patents cited hereinabove that a given inkformulation may interact differently with different types of substrates.Thus, it is quite possible that the quality of the final printed image(being either two-dimensional or three-dimensional) may be significantlyimpaired when the composition of the substrate is changed from one typeto another. In the prior art, methods have been devised that control theinteraction between the ink and the substrate before the ink is finallycured. In particular, the dwell time between the moment of impact of theink on the substrate and the moment of curing is controlled in order toachieve a desired interaction between the substrate and the not-yetcured ink. In this way, the spread of the ink droplets in and over thesubstrate can be controlled in order to acquire a desired dot gain. Theprior art methods however have important disadvantages. For eachcombination of a specific ink and a specific substrate a particulardwell time has to be determined such that an optimal dot gain can beacquired. Given the vast amount of different media types that areavailable today, it will hardly be possible to predetermine dwell timesfor each and every ink-substrate combination. Next to this, even if suchdwell times can be determined beforehand, it will be necessary to devisea printer that can handle dwell times that may vary from a few tenths ofseconds up to several minutes in order to acquire the optimal dot gainfor each and every ink-substrate combination. Such a printer isrelatively expensive and prone to less reliability due to thecomplicated constitution enabling the different dwell times.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or at leastmitigate the above described problems. To this end, a printing methodhas been devised, wherein control is provided for by applying an inkcomprising a carrier composition and an agent that is able to reversiblygel the carrier composition, which agent is soluble in the carriercomposition at the operating temperature of the inkjet print head andgels the ink when it is received on the substrate. The present inventionrelies on the use of an agent that is able to gel the carriercomposition of the radiation curable ink. Gelling the carriercomposition in fact means that the agent thickens the carriercomposition by forming a three-dimensional structure therein. Thecarrier composition is then considered as being a gel. It appears thatby gelling the carrier composition of the ink, the interaction with thesubstrate is dominated by the spreading behavior of the ink as such. Inother words, the type of substrate appears to be only of very minorimportance for the ultimately acquired dot gain. Surprisingly it furtherappears that the type of ink (actually the type of carrier composition)is also of minor importance in the spreading behavior. We have foundthat once the ink is gelled, it is the gel-structure that, in fact,dominates the spreading behavior. This leads to the very favorableresult that ink as well as substrate characteristics no longer dominatethe ultimately acquired dot gain. In practice, this means that the dwelltime can be chosen practically the same for every type of ink-substratecombination. This greatly simplifies the printing method andcorresponding printer.

A gelling agent in general can consist inter alia of high and lowmolecular compounds, a mixture of compounds, or of discrete particles.The molecules or particles of the gelling agent so interact with oneanother that a network is formed in the carrier composition. During thisnetwork formation, it is in principal not necessary for the molecules orparticles of which the gelling agent consists to be actually chemicallybonded or have physical contact. All that is required is that theyshould have a physical interaction such as to result in a reinforcingeffect in the liquid. As a result the viscosity of the liquid increaseswithout it passing over into an actual solid phase. It is noted that itcannot be unambiguously determined beforehand whether a compound canserve as a gelling agent in a carrier composition. This depends on theinteractions between the gelling agent and the carrier composition.Whether an agent can gel a carrier composition has to be determined byexperiment, for example by analytical measurement such as described inU.S. Pat. No. 6,471,758 (see column 8, line 35 to column 9, line 59;with reference to FIGS. 1 to 3).

The present invention makes use of a gelling agent but is restricted tothe use of gelling agents that are soluble in the carrier composition atthe operating temperature of the inkjet print head. This appears to be avery important feature of the present invention. It is believed thatthis contributes significantly to the stability of the ink in the inkjetprint head at the operating temperatures. In general, if the gellingagent forms a second phase in the ink at the operating temperature, thisgives rise to unstable and unpredictable processes when printing the inkfrom an inkjet print head, which typically has multiple miniaturized inkchambers for jetting the ink droplets. In particular, if the gellingagent consists of insoluble particles, there is a tendency of theseparticles to coagulate and clogg the print head ink chambers. Accordingto the present invention, the gelling agent is soluble in the carriercomposition at the operating temperature of the print head and gels theink when it is received on the substrate. Another important aspect ofthe present invention is that the gelling agent gels the ink reversibly.Since in ink jet printing the ink might stay in the print head for arelatively long period, during which the print head is brought severaltimes to its operating temperature (for example every morning at startup), the gel structure that might arise in the ink when the printer isturned off, must be broken when the print head is brought back to itsoperating state.

It is noted that from U.S. Pat. No. 6,467,897 a radiation curable ink isknown containing a thickening agent. This ink however comprises 10-20%of particulate matter that acts as the thickening agent. This ink istherefore almost unsuitable for inkjet printing. From U.S. Pat. No.6,605,652 it is known that a gelling agent can be added to a UV-curableink. However, it is not known from this patent that the gelling agentshould be soluble in the carrier composition at the operatingtemperature of the print head, nor that the gelling agent has to be ableto reversibly gel the ink in order to be practically suitable for inkjetprinting.

In one embodiment of the present invention, the substrate is provided onthe support at a first temperature below the operating temperature ofthe print head. In this embodiment the print head itself is operated ata temperature above the substrate temperature (typicalroom-temperature). This has the advantage that inks can be used thathave a relatively large viscosity at room temperature. The inks namelyare heated to the operating temperature upon which their viscosity isdecreased to enable the drop formation process in the ink jet printhead. The relatively large viscosity at room temperature means thatthere is a further increase in the viscosity of the ink, next to thegelling effect, when the ink is received on the cooler substrate. Thisfurther decreases the influence of ink and substrate characteristics onthe acquired dot gain.

In a further embodiment, the difference between the first temperatureand the operating temperature is at least 30 degrees Celsius. Itsurprisingly appears that when the difference is over 30° C., there is aremarkably larger choice of suitable gelling agents that are soluble inthe carrier composition at the operating temperature of the print head.Next to this, the large difference in temperature enables the use ofcarrier compositions that comprise radiation curable compounds ofrelatively large molecular weight. Large compounds inherently are lessdangerous for the health of people than small compounds. Small compoundsare more volatile and can more easily pass into the body of humans. Thehigher operating temperature provides a low operating viscosity, evenwhen the ink comprises relatively large molecules. Additionally, thegelling process appears to be very fast in this embodiment.

In another embodiment of the present invention, the gelling agent isable to thermally reversible gel the carrier composition. In thisembodiment, the gelling agent is chosen such that the gel structure canbe broken simply by raising the temperature of the ink. This enables areliable use of the ink, without the need of additional means to breakthe gel structure such as ultrasonic probes or other mechanical means.

In a further embodiment, the thermally reversible gelling action isbased on physical interaction between molecules of the gelling agent. Itappears that physical interactions can arise and be broken relativelyfast, which is very advantageous when applying the ink in an inkjetprinter. Next to this, this embodiment ensures a rapid formation of thegel which further decreases the influence of ink and substratecharacteristics as such on the dot gain.

In an even further embodiment, the molecules form a crystalline networkin the carrier composition when the gelling action takes place. Itappears that this further improves the gelling action and thus improvesthe ultimately printing quality to be acquired.

In yet another embodiment the gelling agent is radiation curable. Thisembodiment has the particular advantage that the gelling agent itself isalso curable by the use of radiation. This means that the gelling agentwill become part of the ultimately cured composition, thus stronglyreducing the risk that this agent will migrate in, or even separate fromthe cured ink on the substrate. This greatly improves the durability ofthe ultimate printed images.

In still a further embodiment, the ink received on the substrate issubjected to a physical treatment before it is cured. In this embodimentthe ink is physically treated between the moment of impact on thesubstrate and curing by the influence of radiation. This provides for anopportunity to fine tune the spread of the ink before it is actuallycured. Different levels of spread are, for example, needed depending onthe use of the printed image. This embodiment enables the influence ofthe dot gain without the need of changing the amount or type of gellingagent in the ink. Preferably, the physical treatment comprises thetransfer of heat and/or the application of pressure. It appears that theuse of heat, or pressure or both (for example by using a fuser rollerwell known in the art of printing) adequately enables the fine-tuning ofthe dot gain.

In yet a further embodiment, the print head is scanned over thesubstrate on a scanning carriage, and the ink of a predetermined area ofthe substrate is not cured until the scanning of the said area of thesubstrate is completed. The use of a gelling agent enables the completeprinting of an area of the substrate with the ink, without the need ofintermediate curing. This has a very important advantage. In the priorart namely, intermediate curing between different print stages is oftenapplied to avoid that the ink spreads too much over and in thesubstrate. This means, however, that some ink droplets are jetted onalready cured ink droplets, whereas others are jetted on not yet curedink droplets. This gives rise to local differences in the appearance ofthe printed image, i.e., caused by different degrees of coalescence ofthe droplets, which is very disturbing when high quality images aredesired. In the present embodiment, the interaction between printed (butnot yet cured) ink droplets is consistent over the whole substrate sincenewly jetted ink droplets are always neighbored by the same type ofgelled but uncured ink droplets. This greatly improves the print qualitythat can be achieved by jetting radiation curable inks. In a preferredembodiment the radiation is sent from a source that is mountedseparately from the print head. This avoids the need of scanning theradiation source together with the print head.

The present invention also pertains to a radiation curable inkcomprising a carrier composition and an agent that is able to thermallyreversible gel the carrier composition, which agent is soluble in thecarrier composition at first temperature and gels the carriercomposition when it is cooled to a second temperature below the firsttemperature. Typical gelling agents are high-molecular elongatedmolecules which form an elastic network in a carrier composition, theinterstices of the network being filled with the carrier compositionwhich can be in a liquid or a solid state. If the carrier composition inthe interstices is in a liquid state, a gel arises which has somefluid-like properties, such as the property that molecules can diffuserelatively easily into the continuous liquid matrix and some solid-likeproperties such as the property that the gel can withstand a certainshear stress without deformation occurring, before the gel starts toflow like a liquid. When the liquid in the interstices of the networksolidifies or cures, the gel passes over to the solid state.

In an embodiment of the ink according to the present invention, thegelling agent comprises molecules with a weight-averaged molecularweight of less than 5000, in particular less than 1000, preferably lessthan 500. Since radiation curable inks of themselves are typically moreviscous than water based or other solvent inks, the addition of a smallquantity of well known high-molecular gelling agents such as carragenan,laminarane, pectin and gums such as arabic, xanthane and guar gums canlead to an unacceptably high viscosity at the operating temperature ofthe print head, which means that there is an adverse effect on thejetting properties of the ink. Oligomer gelling agents, i.e., gellingagents with a molecular weight less than 5000 are therefore preferablyused, so that the gelling agent does not have an adverse effect on theviscosity of the ink composition. In a further preferred embodiment,low-molecular gelling agents are used, i.e., gelling agents with amolecular weight less than 1,000 or even 500. The fact that theseoligomer and low-molecular compounds can have gelling properties despitetheir relatively low molecular weight can be explained as follows. Inthe case of oligomer and low-molecular gelling agents, the molecules ofthis agent are believed to separate from the carrier composition on anadequate reduction of the temperature, and form long compound chains viamutual (typical non-covalent) interactions, said chains possiblybehaving in the same way as the high-molecular polymers in thepreviously mentioned well known gelling agents. When the gel is heatedup, the interactions between the molecules of the gelling agent areinterrupted and a sol re-forms. A supplementary advantage of the use ofoligomer and low molecular gelling agents is that the gel-sol transitiontakes place relatively quickly, because for this transition it is onlynecessary to break the relatively weak non-covalent bonds between thecompound molecules of the polymer chains. In addition, small moleculeswill dissolve homogeneously in the carrier composition more rapidly.

In one embodiment, the ink comprises less than 10%, preferably less than5% by weight of the gelling agent. This has the advantage that the agentitself is of less influence on the properties, in particular themechanical properties, of the ultimately cured ink. This is evenstronger in the case where the gelling agent is radiation curable itselfas explained hereinabove.

In another embodiment, an uncured gelled carrier composition de-gels atroom temperature. In this embodiment, the gelling agent is chosen suchthat at room temperature, a gelled but uncured ink slowly de-gels. Thishas the advantage that the ink, on its way from the factory to theclient will not stay in a gelled state. Such a gelled state namely meansthat the ink is very viscous, which means that there are less ways ofgetting the ink out of its container into the printing machine. A slowtransformation in to a normal liquid state at room temperaturepractically ensures that the ink is ungelled when it arrives at aprinter for refilling purposes. Since the process of de-gelling is slow,typically taking from a few hours to several days, the gelling action isstill adequate for providing control of the interaction between ink andsubstrate during the printing process. Thus, the advantages of thepresent invention are fully preserved in this embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in more detail by using thefollowing figures and examples, wherein.

FIG. 1 is a schematic, perspective view showing a portion of a prior artinkjet printing apparatus, wherein the apparatus in this instance is aroll-to-roll vertical inkjet printer;

FIG. 2 is a schematic end elevational view of another prior art inkjetprinting apparatus, wherein the apparatus in this instance is arotatable drum inkjet printer;

FIG. 3 is a schematic, perspective view showing a portion of an inkjetprinting apparatus according to one embodiment of the present invention,wherein the apparatus in this instance is a flatbed inkjet printer; and

FIG. 4 is a schematic, perspective view showing a portion of an inkjetprinting apparatus according to a second embodiment of the presentinvention.

Example 1 defines an ink for use according to the present invention andExample 2 illustrates a second ink for use according to the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates certain components of a prior art inkjet printingapparatus using radiation curable inks. This apparatus has beenextensively described in EP 1 349 733 in paragraphs [0021] to [0043]which paragraphs are incorporated completely herein by reference. Thisapparatus comprises a vertical support plate behind substrate 12, whichsubstrate moves in the upward direction V. An inkjet print head 14extends across the plate and is operable to direct radiation curable inksuch as ultraviolet (UV) curable ink onto the substrate as it movesacross the plate. In practice, the print head 14 is operable to print atleast four inks of different colors so that a wide color spectrum in thefinal printed image can be obtained.

The print head 14 is coupled to a controller 16 for selective activationwhen desired. Controller 16 also controls the movement of the substratedrive system (not shown). The print head 14 is mounted on a carriage 18which is movable in horizontal direction across the width of thesubstrate to print a row of dots of the desired image. The carriage 18is movable along two rails 20 that extend in parallel, in horizontaldirections. A stepping motor 22 is operable to shift the carriage 18along the rails 20. Motor 22 is connected to controller 16 for timed,selective activation of the motor 22 as may be needed. A curing device24 is also mounted on the carriage 18. The curing device 24 includes oneor more sources of radiation, each of which is able to emit light in theultraviolet spectrum. Suitable sources are for example mercury and xenonlamps, carbon arc lamps, tungsten filament lamps, lasers, LED's and thelike. In this particular embodiment the curing device includes onesingle UV lamp 26. The curing device includes a shield (not shown) thatextends substantially over lamp 26 in order to ensure that only theportion of the substrate that lies directly beneath the lamp isirradiated. The curing device is connected to controller 16 and ismounted on the carriage for movement in vertical direction. Steppingmotor 28 is connected to curing device 24 for moving the latter in adirection either away from or towards the print head 14. Controller 16includes a computer for determining a desired dwell time for the inkreceived on the substrate, based on characteristics of the ink andsubstrate used. This dwell time represents the time interval between thetime that the ink is received on the substrate 12 and the time that thesubstrate receives radiation from curing device 24. Once the desireddwell time is calculated, the motor 28 is energized as necessary toshift the curing device 24 either towards or away from the print head14. As an alternative, the dwell time may be varied by changing thespeed of advancement of substrate 12. In this embodiment, motor 28 isnot needed. The disadvantage of this embodiment is that the output speedof the apparatus depends on the particular ink-substrate combination.

FIG. 2 illustrates a prior art printing apparatus 10 a, which comprisesa rotatable drum 11 a. This drum is rotatable around a centralhorizontal reference axis. The drum 11 a is coupled to a transportsystem for moving it around its axis, wherein the rotation is controlledby using controller 16 a. A substrate 12 a is received over the externalsurface of drum 11 a. The apparatus also includes a print head 14 a forjetting radiation curable ink. The apparatus 10 a includes a curingdevice 24 a for directing UV radiation towards ink that is received onthe substrate 12 a. Lamp 26 a of the curing device is connected tocontroller 16 a for activation and deactivation as needed. The curingdevice 24 a is coupled to a pair of guide rails 27 a, one of which isshown in FIG. 2. The rails extend in an arc about the rotational axis ofthe drum 11 a. A motor 28 a is operably connected to the curing device24 a and the rails 27 a for moving the curing device 24 a along therails 27 a as desired. The motor 28 a is also connected to thecontroller for operation.

As can be appreciated by reference to FIG. 2, the motor 28 a is operableto move the UV lamp 26 a in directions either towards or away from theprint head 14 a. As such, the dwell time of the ink received on thesubstrate 12 a can be varied by operation of motor 28 a. Alternatively,the dwell time may be varied by changing the start and stop times of therotational movement of the drum 11 a.

FIG. 3 illustrates certain components of an inkjet printing apparatususing radiation curable inks according to the present invention. In thisembodiment the printer is a flat-bed printer, capable of printing alarge flat panel 12 c. The printer comprises a carriage 18 c that isguided on a rail system 20 c for reciprocal movement in a directionsubstantially perpendicular to the direction F. Carriage 18 c isprovided with eight print heads 14 c, each comprising a different colorin this embodiment, viz. cyan, magenta, yellow and black as well asdiluted versions of each of these colors.

Upon printing an image on the panel 12 c, the carriage is reciprocallymoved over the guide rail 20 c in order to provide one strip of thepanel with rows of ink droplets. In this case, the complete image asintended for this strip is printed. Since the ink contains an agent thatgels the ink droplets as soon as they are received on the substrate,they do not adversely coagulate, spread or bleed into the panel 12 c.After printing of the said strip is completed, the panel is moved in thedirection F such that a next strip neighboring the strip that has justbeen printed faces the print heads 14 c. Then, the next strip of panel12 c is printed with ink droplets. The printer is provided with a curingdevice 24 c, which comprises several lamps (not shown) that are able toemit light in the ultraviolet spectrum towards the panel 12 c. Thecuring device is foreseen with shields 30 that prevent the UV light frombeing scattered over the panel to its surroundings, in particular to theprint heads. When a printed strip with uncured ink passes underneath thecuring device 24 c, radiation is emitted towards the panel, sufficientfor curing the ink located on that strip. Curing device 24 c is astationary device that is not movable with respect to the guide rail 20c. The dwell time of the ink droplets is therefore substantially thesame for all droplets.

FIG. 4 illustrates a second embodiment of a printer according to thepresent invention. The most relevant parts of the printer are depictedin FIG. 4. This printer is provided with a carriage 18 d that isprovided with eight print heads 14 d, the carriage being connected toguide rail 20 d for reciprocal movement across a substrate (not shown).This is the same constitution as is the case with the printer accordingto FIG. 3. The printer comprises a second guide rail 200. To this guiderail carriages 180 and 181 are connected. These carriages are providedwith curing devices 24 d′ and 24 d respectively, each of the curingdevices comprising a mercury lamp (not shown) for emitting radiationtowards the substrate. The carriages can be reciprocated across thesubstrate by corresponding guidance over the rail 200. Optionally theprinter comprises an additional carriage 190 which carries an additionalprint head or set of print heads that is in communication with a sourceof clear ink or other material that lacks color, or even colored ink.The clear ink can be used to improve performance of the finishedproduct, such as by improving durability, gloss, resistance to graffitiand the like. The colored ink, for example white pigmented ink, mightfor example be used to apply spot color having a dedicated customizedtone.

Radiation curable carrier compositions which constitute radiationcurable inks suitable for inkjet printing have been known in the priorart for some time and may comprise for example compositions based onacrylates or thiolenes, which compositions can be cured by providingradicals on emitting radiation. Other known compositions can be cured byproviding cat-ions on emitting radiation to the composition. Mixtures ofvarious compositions are also known in the prior art. Typical radiationtypes that are used for curing include ultraviolet light and electronbeams. UV-curing starts off with initiating a reaction by the emissionof light in the ultraviolet spectrum. A photoinitiating agent absorbsthe UV-light which causes, e.g., the formation of radicals or ions.Electron beam curing is based on the ability of such beams to splitchemical bonds, which causes the formation of free radicals and ions.These particles in turn cause the initiating of a curing reaction. Allthese processes are well known in the prior art. UV-curable inks aretypically based on a carrier composition that comprises acrylatemonomers and oligomers. Well known acrylates in this respect are forexample epoxy acrylates, urethane acrylates, polyester acrylates,silicone acrylates, acrylated dendrimers, polyether acrylates and monoor multiple polyolacrylates. In general the carrier compositioncomprises di- (or even multiple) functional acrylates such that polymerchains (and networks) can be formed in the curing process. The compound1,6-hexanediol-acrylate is commonly applied. Alkoxylated compounds suchas 1,6-hexanediol-ethoxylate-diacrylate are also known for their use inUV-curable inks and have the advantage of being less dangerous to thehealth of mammals. The same is true for dimerised acrylates. It isbelieved that alkoxylated, dimerised acrylates are the least dangerousfor one's health. Next to the acrylates, the composition generallycomprises a photoinitiator. Pigments or other colorants are commonlyused in these inks for enabling marking purposes. Other ingredients thatare commonly known in the art of inks, such as biocides, dispersants,humectants, viscosity modifying agents, surfactants etc. can be appliedas necessary.

Next to the acrylate type radiation curable inks, inks are known thatare based on epoxides, oxetanes and vinyl ethers such as Bisphenol Aepoxides, cycloalipatic epoxides, iso-polyols, alifatic-, aromatic- andalifatic-urethane vinyl ethers. The latter types can also be cured byusing the combination of UV-light and an appropriate photoinitiator(e.g., diaryliodonium or triarylsulfonium salts) which upon absorbingthe UV light generates ions. Carrier compounds used regularly are, forexample, limoneen dioxide, bis{[1-ethyl(3-oxetanil)]methyl}ether,bis-(3-4-epoxycyclohexylmethyl)adipate, 3-ethyl-3-phenoxymethyl-oxetane,3-ethyl-3-[(2-ethylhexyloxy)methyl] oxetane, and3-ethyl-3-hydroxymethyl-oxetane. Photoinitiators commonly used withthese compounds are Esacure 1187 and Chivacure 1172. Manyphotosensitizers are known for these initiators and can be found amongthe anthracene-, xanthone-, thiazine-, acridine-, andporphorine-derivates. Particular compounds are for example 1,6diphenyl-1,3,5 hexatriene, pyrene and perylene.

Radiation curable inks for use in the method according to the presentinvention comprise an agent that can gel the carrier composition whenthe ink is received on the substrate. Herein below, specific examplesare given of inks in which the present invention is embodied.

EXAMPLE 1

A UV-curable ink has been made that is based on a carrier compositioncomprising 36,5 weight % 1,6-hexanediol-ethoxylate-diacrylate (seeFormula 1 here-beneath), 36,5 weight % di-trimethylolpropanetetraacrylate (see Formula 2), 18 weight % N-vinylcaprolactam (seeFormula 3) and 9 weight % of the photoinitiator 2 paratolyl-2-(dimethylamino)-4′-morfolinobutyrofenon available (at CibaSpecialty Chemicals, Basel, Switzerland) under the tradename Irgacure379. To this carrier composition 1.5 parts per hundred parts carriercomposition (1.5 phr) carbon black is added as a marking material. Thiscarbon black is available as Nipex 150 at Degussa AG Germany, and isdispersed using Solsperse 39000 (1 part per part carbon black) andSolsperse 5000 (1 part per 4 parts Carbon black) both available fromNoveon Inc., USA with 2-butanone as the dispersing medium. Next to this1.5 phr stearon (i.e., (C₁₇H₃₅)₂C═O) is added as a gelling agent. Notethat stearone as a compound for inkjet inks is known as such. However,in combination with this particular carrier composition it acts as agelling agent which is hitherto unknown. Lastly 500 ppmmethoxyhydroquinon is added as an inhibitor.

This ink can be used at an operating temperature of 70° C. (for examplein the printers according to FIGS. 3 and 4), at which temperature thestearone is just dissolved in the carrier composition. If needed, someexcess stearone that does not dissolve at the operating temperature canbe removed by filtration before the ink is used in the inkjet printinghead. If the ink is jetted onto a substrate that has a temperature ofabout 25° C., the ink will rapidly transform into a gelled state. Thisavoids too much spread, coalescence between neighboring ink droplets andbleed into the substrate. Other ink formulations based on the samecarrier composition were made by using, as a gelling agent,octadecane-amide, stearylstearamide and Ceridust TP 5091 (available atClariant, Muttenz, Switzerland) The latter gelling agent is UV curableand will cure together with the carrier composition.

EXAMPLE 2

A second UV-curable ink has been made that is based on a carriercomposition comprising 4.9 weight % Limoneen dioxide (LDO) (availablefrom Arkema Inc, Philadelphia, USA), 24.9 weight %Bis{[1-ethyl(3-oxetanil)]methyl}ether (available as OXT-221 fromDKSH-Market Intelligence, Zurich, Switzerland), 20.0 weight %Bis-(3-4-epoxycyclohexylmethyl)adipate (available as UVR-6128 from DowChemicals, Horgen, Switzerland), 32.35 weight % (m/m)3-ethyl-3-phenoxymethyl-oxetane (available as OXT-211 from DKSH-marketintelligence, Zurich, Switzerland). The carrier composition furthercomprises 1.25 weight % Carbon black, 1.5 % 2-ethyl-9,10-dimethoxyanthracene (available from Sigma-Aldrich, St.-Louis, USA), 15% Photoinitiator Chivacure 1172 (available from Double Bond Chemical, TapeiTaiwan) and 0.1 % Byk UV3510 (available from Byk Chemie GmbH, Wesel,Germany). As a gelling agent 2 phr stearon is added. This ink can beused in the same way as the ink described under example 1.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A method for printing a substrate with radiation curable ink whichcomprises providing the substrate on a support, providing an inkjetprint head at an operating temperature, jetting droplets of the curableink from the inkjet print head onto the substrate, controlling theinteraction between the ink and the substrate, and then curing the inkreceived on the substrate by radiating the substrate, wherein the saidinteraction is controlled by applying an ink comprising a carriercomposition and an agent that is able to reversibly gel the carriercomposition, which agent is soluble in the carrier composition at theoperating temperature of the inkjet print head and gels the ink when itis received on the substrate such that the ink droplets sufficientlyspread over the substrate to provide an adequate interaction with thesubstrate.
 2. The method according to claim 1, wherein the substrate isprovided on the support at a first temperature below the operatingtemperature.
 3. The method according to claim 2, wherein the differencebetween the first temperature and the operating temperature is at least30 degrees Celsius.
 4. The method according to claim 1, wherein theagent is able to thermally reversibly gel the carrier composition. 5.The method according to claim 4, wherein the thermally reversiblegelling action is based on physical interaction between molecules of thegelling agent.
 6. The method according to claim 5, wherein the moleculesform a crystalline network in the carrier composition when the gellingaction takes place.
 7. The method according to claim 1, wherein thegelling agent is radiation curable.
 8. The method according to claim 1,wherein the ink received on the substrate is subjected to a physicaltreatment before it is cured.
 9. The method according to claim 8,wherein the physical treatment comprises the transfer of heat and/or theapplication of pressure.
 10. The method according to claim 1, whereinthe print head is scanned over the substrate on a scanning carriage, andthe ink of a predetermined area of the substrate is not cured until thescanning of the said area of the substrate is completed.
 11. The methodaccording to claim 10, wherein the radiation is sent from a source thatis mounted separately from the print head.
 12. A radiation curable inkcomprising a carrier composition and an agent that is able to thermallyreversible gel the carrier composition, said agent being soluble in thecarrier composition at a first temperature and gels the carriercomposition when it is cooled to a second temperature below the firsttemperature.
 13. The ink according to claim 12, wherein the gellingagent comprises molecules with a weight-average molecular weight of lessthan 5000, in particular less than 1000, preferably less than
 500. 14.The ink according to claim 12, wherein the ink comprises less than 10%,preferably less than 5% by weight of the gelling agent.
 15. The inkaccording to claim 12, wherein the gelling agent is a crystallinecompound.
 16. The ink according to claim 12, wherein the gelling agentis radiation curable.
 17. The ink according to claim 12, wherein anuncured gelled carrier composition de-gels at room temperature.
 18. Aninkjet printer adapted to carry out the method according to claim 1 19.A carrier composition containing a gelling agent which is soluble in thecarrier composition of the operating temperature of ink print heads; hasthe ability to thermally, reversibly gel the carrier composition andacts to thicken the carrier composition.
 20. A carrier compositioncontaining stearon as a gelling agent, said carrier compositioncomprising 1,6-hexanediol-ethoxylate-diacrylate, di-trimethylolpropanetetraacrylate and N-vinylcaprolactam.
 21. A radiation curable inkcomposition which comprises a radiation curable ink containing thecarrier composition of claim 20.